An electrosurgical generator is used in surgical procedures to deliver electrical energy to the tissue of a patient. An electrosurgical generator often includes a radio frequency generator and its controls. When an electrode is connected to the generator, the electrode can be used for cutting or coagulating the tissue of a patient with high frequency electrical energy. During normal operation, alternating electrical current from the generator flows between an active electrode and a return electrode by passing through the tissue and bodily fluids of a patient.
The electrical energy usually has its waveform shaped to enhance its ability to cut or coagulate tissue. Different waveforms correspond to different modes of operation of the generator, and each mode gives the surgeon various operating advantage. Modes may include cut, coagulate, a blend thereof, desiccate, or spray. A surgeon can easily select and change the different modes of operation as the surgical procedure progresses.
In each mode of operation, it is important to regulate the electrosurgical power delivered to the patient to achieve the desired surgical effect. Applying more electrosurgical power than necessary results in tissue destruction and prolongs healing. Applying less than the desired amount of electrosurgical power inhibts the surgical procedure. Thus, it is desirable to control the output energy from the electrosurgical generator for the type of tissue being treated.
Different types of tissues will be encountered as the surgical procedure progresses and each unique tissue requires more or less power as a function of frequently changing tissue impedance. Even the same tissue will present a different load impedance as the tissue is desiccated.
Two conventional types of power regulation are used in commercial electrosurgical generators. The most common type controls the DC power supply of the generator by limiting the amount of power provided from the AC mains to which the generator is connected. A feedback control loop regulates output voltage by comparing a desired voltage with the output voltage supplied by the power supply. Another type of power regulation in commercial electrosurgical generators controls the gain of the high-frequency or radio frequency amplifier. A feedback control loop compares the output power supplied from the RF amplifier for adjustment to a desired power level. Generators that have feedback control are typically designed to hold a constant output voltage, and not to hold a constant output power.
U.S. Pat. Nos. 3,964,487; 3,980,085; 4,188,927 and 4,092,986 have circuitry to reduce the output current in accordance with increasing load impedance. In those patents, constant voltage output is maintained and the current is decreased with increasing load impedance.
U.S. Pat. No. 4,126,137 controls the power amplifier of the electrosurgical unit in accord with a non linear compensation circuit applied to a feedback signal derived from a comparison of the power level reference signal and the mathematical product of two signals including sensed current and voltage in the unit.
U.S. Pat. No. 4,658,819 has an electrosurgical generator which has a microprocessor controller based means for decreasing the output power as a function of changes in tissue impedance.
U.S. Pat. No. 4,727,874 includes an electrosurgical generator with a high frequency pulse width modulated feedback power control wherein each cycle of the generator is regulated in power content by modulating the width of the driving energy pulses.
U.S. Pat. No. 3,601,126 has an electrosurgical generator having a feedback circuit that attempts to maintain the output current at a constant amplitude over a wide range of tissue impedances.
None of the aforementioned U.S. Patents include a constant power control circuit that provides for a generally constant output power while also providing a linear adjustment to account for the unique waveform crest factors associated with different operational modes.
The preferred constant power control circuit and method provided herein allows for output power control by way of a unique and simple linear conversion circuit coupled with protection circuitry that prevents the electrosurgical generator from being over-driven during high and/or low impedance loading. The preferred constant power control circuit also reduces the severity of exit sparking by responding quickly to high impedance indications while nonetheless maintaining substantially increased power levels throughout a predetermined patient tissue impedance range.